1. Field of the Invention
The present invention relates to a communication method suitable for application to a radio telephone system, for example, and a base station and a terminal apparatus to which the communication method is applied.
2. Description of the Related Art
In a mobile communication such as a radio telephone system or the like, a multiple access in which a plurality of mobile stations (terminal apparatus or subscribers) are permitted to access a single base station is employed. In this case of a radio telephone, a number of mobile stations commonly utilize a single base station. Therefore, various communication systems have been proposed for avoiding interference between respective mobile stations. For example, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, a code division multiple access (CDMA) system and so on are conventionally proposed as this kind of communication systems.
Of these systems, the CDMA system is a multiple access system in which a particular code is assigned to each of the mobile stations, a modulated wave of an identical carrier wave (carrier) is spread in spectrum with the code and then transmitted to the identical base station, and a base station receives it with taking code synchronism based on each code to identify a desired mobile station.
Specifically, the base station occupies the whole frequency band owing to the spectrum, and transmits signals to a plurality of mobile stations using an identical frequency band at the same time. Each of the mobile stations inversely spreads a signal of a fixed spread band width transmitted from the base station to extract a corresponding signal. Further, the base station discriminates each of the mobile stations by different spread codes one another.
In the CDMA system, communication can be achieved at every direct calling so long as a code is shared. Further, the system is excellent in secrecy of telephone conversation. Therefore, the system is suitable for a radio transmission utilizing mobile stations such as a portable telephone apparatus and so on.
In the CDMA system, it is difficult to establish a precise communication relationship between mobile stations. Therefore, each communication between respective mobile stations cannot be dealt completely separately, and hence another mobile station can become a source of interference upon communication with a mobile station. Further, data is spread within a particular frequency band in this system. Therefore, it is necessary to define a band width in advance over which the data is spread (i.e., a band width for use of transmission). Therefore, it is difficult to change the transmission band width.
The above matter will be described more concretely. FIGS. 1A and 1B shows a model in which a transmission signal of a particular user is extracted by an inverse spread from transmission signals of eight mobile stations (users) which are spectrum spread with predetermined codes and multiplexed, for example. As shown in FIG. 1A, if a signal of a user U0 is to be extracted by the inverse spread from signals of eight users U0 to U7 which are multiplexed with codes, then as shown in FIG. 1B, the signal of the user U0 can actually be extracted. However, signals of other users U1 to U7 which are dealt by the same base station also become an interference source, serving as a noise. This fact results in deterioration of an S/N characteristic. For this reason, in a radio transmission employing the CDMA system, electric wave does not reach well due to the deterioration caused by the interference, which fact narrows a service area. Further, interference due to other users can be suppressed only by an amount of inverse spread gain which is obtained in a process of spectrum inverse spread. Therefore, a number of users (mobile stations) permitted to access is limited and a channel capacity becomes small.
In the communication system in which the multiple access of this kind is carried out, it is important in suppressing interference of other user to set transmission powers of respective transmission signals which simultaneously exist within a certain range. However, in the communication system carrying out the multiple access such as the conventional CDMA system or the like, the processing for controlling the transmission power is not always satisfactorily carried out.
Specifically, in order to set a transmission power from a certain terminal apparatus within a certain range, the base station receives a signal transmitted from the terminal apparatus to detect a transmission status of the signal and transmits a control data for the transmission power based on the detection result to the terminal apparatus. The terminal apparatus determines the transmitted control data to carry out the processing for adjusting the transmission power to be within the state the control data indicates.
FIG. 2 shows an arrangement for the control used when a power of an upward channel from the terminal apparatus to the base station. In a base station 910, a demodulating circuit 911 demodulates a signal transmitted from a terminal apparatus 920, and a power control information generating unit 912 generates a power control information based on the demodulation result (e.g., an estimation value of a reception symbol error). A multiplexing circuit 913 multiplexes the power control information together with a transmission symbol from the base station in accordance with a predetermined method. A modulating circuit 914 modulates the multiplexed signal. A transmission amplifier 915 subjects the modulated signal to a transmission processing and transmits the signal to the terminal apparatus 920 by wireless.
In the terminal apparatus 920, a demodulating circuit 921 demodulates a received signal of a channel from the base station. A separating circuit 922 separates the power control information included in the demodulation signal from the reception symbol and then supplies the separated power control information to a transmission power control unit 923. The transmission power control unit 923 controls a gain of a transmission amplifier 924 to be a value corresponding to a desired power.
Since the control loop is arranged as described above, it becomes possible to satisfactorily control the transmission output based on a practical transmission state. It is possible to cancel difference among the transmission powers in respective paths.
In the conventional processing, since an actual circuit quality is different from a detected circuit quality, even if the transmission power is controlled up to a certain range to align the circuit qualities, then it is difficult to control the circuit quality to be strictly set within a certain range.
For example if an estimation values of a reception symbol error is calculated from the demodulation result in the processing shown in FIG. 2 to estimate a circuit quality based on the estimation value of the reception symbol error, then the result shown in FIG. 3 is obtained. Specifically, an estimation value Qa of the circuit quality changed in response to the actual circuit quality Q.sub.0 shown by a solid line in FIG. 3 is obtained. However, there is a certain delay between the actual circuit quality Q.sub.0 and the detected circuit quality estimation value Qa. Therefore, a response of the control loop shown in FIG. 2 becomes worse.
In a processing for estimating a circuit quality based on another method, the circuit quality can be estimated from a reception power. For example, as shown in FIG. 4, a reception signal which is not demodulated by the demodulating circuit 911 is supplied to a reception power measuring circuit 916. A reception power value measured by the reception power measuring circuit 916 is supplied to a comparator 917. The comparator 917 compares the value with a reception power set value output from a set value generating circuit 918. A power control information is generated based on the comparison result and multiplexed by a multiplexing circuit 913 together with the transmission symbol, from the base station in accordance with a predetermined method. The multiplexed signal is modulated by a modulating circuit 914. A transmission amplifier 915 and so on subject the modulated signal to the transmission processings and then transmits the signal to the terminal apparatus 920.
When the circuit quality is estimated from the reception power as described above, the estimation result is as shown in FIG. 5. Specifically, a quality estimation value Qb calculated from an average of the reception powers P is changed substantially similarly to the actual circuit quality Q.sub.0, but the quality estimation value has a difference y from the actual quality depending upon a status of a transmission path (fading, multipath, interference wave and so on). This difference y is different depending upon the status of the transmission path, and hence it is difficult t precisely estimate the circuit quality.